Widespread plant species are expected to maintain genetic diversity and gene flow via pollen and seed dispersal. Stature is a key life history trait that affects seed and potentially pollen ...dispersal, with limited stature associated with limited dispersal and greater genetic differentiation. We sampled Hill’s tabletop wattle (Acacia hilliana) and curry wattle (Acacia spondylophylla), two co‐distributed, widespread, Acacia shrubs of low stature, across the arid Pilbara region of north‐western Australia. Using chloroplast sequence and nuclear microsatellite data we evaluated patterns of population genetic and phylogeographic diversity and structure, demographic signals, ratios of pollen to seed dispersal, evidence for historical refugia, and association between elevation and diversity. Results showed strong phylogeographic (chloroplast, GST = 0.831 and 0.898 for A. hilliana and A. spondylophylla, respectively) and contemporary (nuclear, FST = 0.260 and 0.349 for A. hilliana and A. spondylophylla, respectively) genetic structure in both species. This indicates limited genetic connectivity via seed and pollen dispersal associated with Acacia species of small stature compared to taller tree and shrub acacias across the Pilbara bioregion. This effect of stature on genetic structure is superimposed on moderate levels of genetic diversity that were expected based on widespread ranges (haplotype diversity h = 25 and 12; nuclear diversity He = 0.60 and 0.47 for A. hilliana and A. spondylophylla, respectively). Contemporary genetic structure was congruent at the greater landscape scale, especially in terms of strong genetic differentiation among geographically disjunct populations in less elevated areas. Measures of diversity and connectivity were associated with traits of greater geographic population proximity, population density, population size, and greater individual longevity, and some evidence for range expansion in A. hilliana. Results illustrate that low stature is associated with limited dispersal and greater patterns of genetic differentiation for congenerics in a common landscape and highlight the complex influence of taxon‐specific life history and ecological traits to seed and pollen dispersal.
We assessed phylogeographic and population genetic diversity and structure in two Acacia shrubs of low stature with widespread ranges and co‐distributions across the arid Pilbara region of north‐western Australia: Hill’s tabletop wattle (Acacia hilliana) and curry wattle (Acacia spondylophylla). Both species showed strong phylogeographic and contemporary genetic structure indicating reduced genetic connectivity when compared to taller shrub and tree Acacias in the Pilbara but maintained levels of genetic diversity comparable to that of other widespread Acacia
AIM: Fire regimes are a significant driver of plant adaptation and evolution, and projected changes under a changing climate will have significant implications for demographic and evolutionary ...responses of plant populations in fire‐prone regions globally. We tested predictions regarding level and pattern of genetic variation within a model post‐fire facultative seeder, Bossiaea ornata (Fabaceae), that exhibits a combination of both post‐fire resprouting and seeding strategies. LOCATION: Fire‐prone forests in the mediterranean‐climate region of south‐western Australia. METHODS: We genotyped individuals with eight microsatellite markers and four non‐coding chloroplast sequence regions to assess population genetic variation and reconstruct phylogeographical history. RESULTS: Extremely high levels of chloroplast (HD = 0.95) and nuclear (HE = 0.81) diversity were detected. Nuclear population differentiation was weak (FST = 0.06) and chloroplast phylogeographical structure was absent. A significant excess of rare chloroplast haplotypes was detected (D, FS, R₂; all P < 0.01), consistent with demographic expansion. MAIN CONCLUSIONS: Results suggest that a facultative seeding life‐history affords the benefits of both obligate resprouting and obligate seeding strategies in terms of resistance and resilience to fire. High diversity and low differentiation are likely driven by long generational overlap (resprouter feature), regular post‐fire population supplementation from a persistent soil seed bank (seeder feature) and long‐term stability. Expansion signals may be a feature of species with regular, synchronous post‐fire recruitment from a soil seed bank, rather than evidence of a single range or demographic expansion event, as typically interpreted. The facultative seeding strategy may therefore offer resilience to potential increases in fire frequency under a changing climate in fire‐prone regions.
Phylogeographic studies can be used as a tool to understand the evolutionary history of a landscape, including the major drivers of species distributions and diversity. Extensive research has been ...conducted on phylogeographic patterns of species found in northern hemisphere landscapes that were affected by glaciations, yet the body of literature for older, unaffected landscapes is still underrepresented. The Pilbara region of north-western Australia is an ancient and vast landscape that is topographically complex, consisting of plateaus, gorges, valleys, and ranges, and experiences extreme meteorological phenomena including seasonal cyclonic activity. These features are expected to influence patterns of genetic structuring throughout the landscape either by promoting or restricting the movement of pollen and seed. Whilst a growing body of literature exists for the fauna endemic to this region, less is known about the forces shaping the evolution of plant taxa. In this study we investigate the phylogeography of two iconic Pilbara tree species, the Hamersley Bloodwood (
) and Western Gidgee (
), by assessing patterns of variation and structure in several chloroplast DNA regions and nuclear microsatellite loci developed for each species. Gene flow was found to be extensive in both taxa and there was evidence of long-distance seed dispersal across the region (pollen to seed ratios of 6.67 and 2.96 for
and
, respectively), which may result from flooding and strong wind gusts associated with extreme cyclonic activity. Both species possessed high levels of cpDNA genetic diversity in comparison to those from formerly glaciated landscapes (
= 14 haplotypes,
= 37 haplotypes) and showed evidence of deep lineage diversification occurring from the late Miocene, a time of intensifying aridity in this landscape that appears to be a critical driver of evolution in Pilbara taxa. In contrast to another study, we did not find evidence for topographic features acting as refugia for the widely sampled
.
Aim
Apomixis is a widespread trait in extreme environments worldwide, yet phylogeographical studies for species exhibiting these complex reproductive systems are still limited to temperate zones in ...the Northern Hemisphere. Through analyses of a combination of adult plants and seedlings, and nuclear and chloroplast DNA, we assessed the contemporary genetic outcomes of apomixis and phylogeographical patterns in an arid unglaciated landscape to understand the evolutionary trajectory of apomictic species.
Location
Pilbara bioregion, north‐western Australia.
Taxon
Senna glutinosa subsp. glutinosa.
Materials and Methods
Spatial patterns of genetic diversity and differentiation in three chloroplast regions and 17 nuclear microsatellite markers were evaluated in 480 plants from 20 populations of S. glutinosa subsp. glutinosa across the Pilbara bioregion. We also germinated and genotyped seed for a progeny analysis to quantify polyembryony and the relative frequency of asexual versus sexual reproduction.
Results
We found frequent polyembryony and a high proportion of asexual offspring (84.56%), resulting in low within‐population genotypic diversity. The nuclear dataset identified 72 multilocus lineages (MLLs) with moderate pairwise population genetic fixation and low differentiation among populations with no geographically structured genetic clustering. Chloroplast DNA showed high haplotype diversity and a widespread distribution of haplotypes. Only one peripheral population presented a single unique haplotype and MLL.
Main Conclusions
Analysis of this woody perennial in an arid landscape shows the influence of apomixis on genetic patterns and species persistence. Haplotype diversity was indicative of long‐term persistence within the ancient Pilbara region. Widespread distribution of haplotypes and sharing of several nuclear MLLs among distant populations indicated extensive seed dispersal throughout the study area. The combination of polyploidy, facultative apomixis and extensive seed dispersal maintains genetic variability within local populations and promotes the spread of advantageous genotypes across wide geographical distances. Therefore, apomixis, rather than an evolutionary ‘dead‐end’, can be regarded as a mechanism facilitating evolutionary success of apomictic species in extreme and complex environments.
Understanding how genetic diversity is distributed and maintained within species is a central tenet of evolutionary and conservation biology, yet is understudied in arid regions of the globe. In ...temperate, glaciated environments, high genetic diversity in plant species is frequently found in refugial areas, which are often associated with southern non-glaciated landscapes. In arid, unglaciated environments, landscape features providing mesic conditions are likely to be refugia, although our understanding needs more refinement in these biomes. We test whether refugia and nuclear diversity hotspots occur in high-elevation, topographically complex areas for co-distributed shrubs (Petalostylis labicheoides and Indigofera monophylla; Fabaceae) in the ancient, arid Pilbara bioregion of north-western Australia.
We conducted extensive sampling of the Pilbara (>1400 individuals from 62 widespread populations) to detect patterns in nuclear diversity and structure based on 13-16 microsatellite loci. Evidence of historical refugia was investigated based on patterns of diversity in three non-coding chloroplast (cp) sequence regions for approx. 240 individuals per species. Haplotype relationships were defined with median-joining networks and maximum likelihood phylogenetic trees.
We found cpDNA evidence for a high-elevation refugium in P. labicheoides but not for I. monophylla that instead exhibited extraordinary haplotype diversity and evidence for persistence across a widespread area. Nuclear diversity hotspots occurred in, but were not exclusive to, high-elevation locations and extended to adjacent, low-elevation riparian areas in both species.
Phylogeographic refugia in arid environments may occur in high-elevation areas for some species but not all, and may be influenced by species-specific traits: a mesic montane refugium in P. labicheoides could be related to its preference for growth in water-gaining areas, while a lack of such evidence in I. monophylla could be related to maintenance of cpDNA diversity in a large soil seed bank and dynamic evolutionary history. Mesic environments created by the intersection of topographically complex landscapes with riparian zones can be contemporary reservoirs of genetic diversity in arid landscapes.
AIM: Terrestrial plant ecology and evolution is significantly influenced by the phenomenon of fire, but studies of its potential impact on intraspecific genetic variation and phylogeography are rare. ...This understanding will be important for predicting the biogeographical consequences of changing fire regimes under global climate change. Here, we asked whether changing historical fire regimes, together with climatic and geological history, have influenced phylogeographical patterns in a fire‐ephemeral vine. We also asked whether demographic stochasticity associated with a fire‐ephemeral life history results in nuclear genetic drift as expected from spatio‐temporal patchiness, or if this effect is buffered by the connectivity and diversity afforded by a persistent soil seed bank. LOCATION: The fire‐prone, mediterranean‐type climate region of south‐western Australia. METHODS: We used Bayesian phylogeny reconstruction and statistical tests of demographic expansion based on variation at three non‐coding chloroplast sequence regions (atpF, ndhF–rpl32, psbD–trnT) to reconstruct phylogeographical history. Nuclear diversity and population structure at 11 microsatellite loci were investigated for evidence of genetic drift. RESULTS: Evidence for prolonged persistence and a lack of vicariance within the species range was found, together with strong evidence of historical demographic expansion. Contrary to expectations, there was little evidence of nuclear genetic drift despite strong, above‐ground spatio‐temporal population patchiness. MAIN CONCLUSIONS: Our findings suggest that a late Pleistocene increase in fire frequency may have led to demographic expansion in this fire‐ephemeral species; alternatively, the expansion signal may be an inherent feature of fire ephemerals with a persistent soil seed bank. Prolonged climatic stability has likely fostered persistence within the species range in contrast to contraction and vicariance. The notable lack of genetic drift implies a role for ample pollen dispersal and a long‐lived soil seed bank in the maintenance of diversity and connectivity in an otherwise stochastic, fire‐driven system.
The degree of clonality, interspecific hybridisation and inbreeding in rare species will have significant implications for maintenance of genetic diversity and resilience to environmental change, ...with related implications for immediate conservation management. Using microsatellites, we detected extensive clonality within the rare mallee, Eucalyptus absita, from a highly disturbed agricultural habitat in southwestern Australia. A total of just 16 unique genets, plus five putative hybrids with E. loxophleba, were detected across the known species range and these genets were estimated to be of considerable age. Each location possessed a unique genotype and overall diversity was moderate (H E = 0.547). The outcrossing rate in seedling progeny was low (t ₘ = 0.281), reflecting a rarity of intraspecific sires and minimal inbreeding depression prior to seed dispersal. Of all seedling progeny, up to 19 % were putative hybrids. Our findings indicate that despite rarity and clonality, moderate levels of genetic diversity and the capacity to produce outcrossed seeds is maintained. However, the ongoing maintenance of E. absita genetic diversity is significantly compromised by a high rate of selfing and potential hybridisation in seedling progeny. Seeds collected for long-term storage or rehabilitation should be screened for inbreeding and hybridisation rates to improve conservation outcomes. All existing adults represent a unique portion of the genepool for conservation.
Aim: Uniform spatial population distributions are predicted to result in lower among-population genetic differentiation and higher within-population genetic diversity than naturally patchy ...distributions, but there have been surprisingly few attempts to isolate this effect from confounding factors. We studied the widespread wind-pollinated shrub Allocasuarina humilis that is common in a geologically stable landscape characterized by long-term population persistence to test the influence of semi-continuous versus patchy population distributions on genetic patterns. We also investigated whether A. humilis shows the high population connectedness and genetic diversity typically associated with wind pollination, a relatively uncommon and little-studied syndrome in this landscape. Location: Heath-shrublands ('heath') and forests of south-western Australia. Methods: Populations were sampled from heath and forest regions, which respectively exhibited semi-continuous and patchy population distributions. Genetic structure and diversity were assessed for 27 populations using eight nuclear microsatellite markers and three chloroplast regions. Phylogeographical history was examined using Bayesian phylogeny reconstruction, parsimony analysis and tests of expansion. Results: High haplotype diversity is consistent with long-term population persistence across most of the species' range. Nuclear markers showed low overall population differentiation and no geographical structure over c. 900 km, reflecting extensive pollen dispersal. For both marker types, patchily distributed forest populations were substantially more differentiated with significantly lower within-population diversity than semi-continuous heath populations. Phylogeographical analysis revealed evidence for earlier colonization of heath than forest and recent expansion into wetter forests, consistent with progressive long-term climatic drying. Main conclusions: High population connectedness and genetic diversity probably resulted from wind pollination in combination with dioecy and long life span. Patchy population distributions appear to have influenced genetic structure and diversity through lower pollen and seed dispersal, lower effective population sizes and greater genetic drift. Our approach illustrates the value of minimizing confounding variables by testing the effect of a variable ecological trait within a single species.
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